<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<div id="box">
    <input type="text" v-model="num1">
    <select v-model="option">
        <option value="0">+</option>
        <option value="1">-</option>
        <option value="2">*</option>
        <option value="3">/</option>
    </select>
    <input type="text" v-model="num2">
    <input type="button" value="=" @click="run">
    <input type="text" v-model="num3" readonly="readonly">
</div>

<script src="vue.js"></script>
<script>
    var vm = new Vue({
        el: '#box',
        data: {
            num1: '',
            num2: '',
            num3: '',
            option: '0'
        },
        methods: {
            run: function () {
                switch (this.option) {
                    case '0':
                        this.num3 = (this.num1 - 0) + (this.num2 - 0);
                        break;
                    case '1':
                        this.num3 = this.num1 - this.num2;
                        break;
                    case '2':
                        this.num3 = this.num1 * this.num2;
                        break;
                    case '3':
                        this.num3 = this.num1 / this.num2;
                        break;

                }
            }

        }
    })
</script>
</body>
</html>